1. Field of the Invention
The invention relates to a screw implant for a jawbone comprising a threaded conical body which can be screwed into the jawbone, on which is formed an implant neck and a conical implant post to accept a replacement tooth or a substructure.
2. Description of Related Art
There are many forms of known tooth implants. Only a few of these have found practical application. For example, needle implants and blade or plate and screw implants have been used. Even though screw implants have been found to be especially useful, they have not fulfilled expectations. Proposed forms of such implants have been found to be deficient, especially because of insufficient anchoring, damage to the bone, or because of complicated insertion methods.
German Offenlegungschrift 22 55 916 (May 16, 1974) describes a screw implant which has a helical thread as well as a substantially constant outside diameter. Good initial anchoring is taught to be achievable and the anchoring effect is intended to improve as time progresses. However, it has been found that, since the diameter of the shaft is relatively large, the thread of the shaft cuts into the bone bed during implantation such that destruction of bone tissue cannot be avoided Voids formed in the threads have an adverse influence on the lifetime of the implant, and also form infection sites. Furthermore, the rigid screw-in implant cannot be bent, as for straightening, after implantation. As a rule, it is only possible to shorten such an implant by grinding of the column or post carried by the implant neck. This is especially unpleasant for the patient, since the grinding has to be done after the insertion of the implant.
Another embodiment of a screw implant as described in Grafelmann, et al. U.S. Pat. No. 4,406,623 (Sept. 27, 1983) has a conical screw shaft with a helical thread. The purpose of this structure is to impart to the implant enhanced strength in the implanted state. It is clear from this patent that, in screw implants, the form of the thread is of special importance, and very precise data are given. V-shaped sections are cut out from the threads of the helical thread winding at angular distances which are smaller than 360.degree. but not equal to 180.degree.. These cuts are radially directed and, starting from the periphery of the helix, the tips are in the region of the shaft surface. Furthermore, sharpened parts which extend in the direction of insertion are formed. Thus, a very sharply cutting thread is provided and such an implant cuts into the bone relatively deeply. However, this results in bone tissue destruction and development of void spaces. Moreover, straightening of the implant is not possible and here, too, grinding processes must be employed for adaptation.
German Offenlegungschrift 26 00 639 (July 15, 1976) describes an insert for a tooth bed wherein the insert has a nonbendable strengthening extension and a thread, with the threads being cut deeply in such a way that, upon insertion, voids are formed between the insert and the tissues which may lead to inflammation.
A screw-type jaw implant made from ceramic materials is described in Munch U.S. Pat. No. 4,468,200 (Aug. 28, 1984). At a lower end, which is intended for anchoring in the jawbone, a conical screw having a rounded end is provided, and at its upper end is a hollow groove for attachment of the epithelial sleeve and has an opening to accept a tooth support. This is designed for insertion immediately after a tooth is extracted. There is an annular thickened portion between the upper and lower ends, which is designed as a conical screw and there are at least two annular notches, whereby the diameter of the annular thickened region is smaller than the diameter of the thread peaks at the uppermost thread of the lower part of the jaw implant. In order to achieve an especially secure anchoring, the lower part of the jaw implant is provided with sawtooth-like threads but a disadvantage of these is that the jaw tissue is damaged when the implant is inserted. The thread design does not permit elimination of voids. Moreover, the securing section for holding the replacement tooth cannot be adjusted.
It has long been known that an implant must be anchored permanently. In addition to eliminating or minimizing void spaces in the bone tissue, the implant should be supported at both ends in the bone, that is, it must be supported by the compact bone tissue of the alveolar countercorticalis. However, as a rule, the formation of voids can only be prevented when, during the insertion of the screw implant, there is no destruction of the bone tissue through which it travels. In the case of prior known screw implants, this cannot be prevented due to the relatively deep penetration of the cutting threads. However, as a result of the destruction of tissue and the development of voids around the implant, primary stability, on which later retention depends and which is responsible for a strong bond between the implant and bone, is not ensured. Especially in the front tooth region of the lower jaw, the length of the screw implant determines the degree of support by the bone tissue against which the implant lies during insertion. Therefore, either an implant with an accurate length must be used, or the end that protrudes from the jaw (i.e. the implant post) must be shortened or ground to the proper size.